Underreamers are tools used for enlarging the gage of a hole drilled into the earth such as an oil or gas well. A typical underreamer has a tubular body which can be connected to a drill string. One or more cutting arms are housed in the tubular body as the underreamer is lowered through a borehole. When the region to be underreamed to a larger diameter is reached, the cutter arms are deployed outwardly away from the body to engage the gage of the drill hole. Hard cutting surfaces carried on the cutting arms engage the rock formation and crush it away as the underreamer is turned by the drill string. A variety of cutters can be mounted on the arms.
It is desirable to establish boreholes having different diameters for different purposes. It is accepted practice to drill an initial hole with standardized drilling equipment and later ream out the hole under a restriction to the desired gage with underreamers.
The diameter of an underreamed hole is established by limiting the outward movement of the cutting arms from the underreamer body. Each cutting arm has a particular size and opening diameter. Previously, every time the reaming gage was changed, one replaced the cutting arms from the underreamer with a new set of arms having the desired extension. This approach is time consuming and requires a large inventory of cutting arms.
It would be desirable to provide an underreamer which has a readily adjustable cutting arm extension.
One underreamer arrangement is disclosed in U.S. Pat. No. 4,282,941, issued Aug. 11, 1981, to Perkin. In this underreamer, each cutter support arm is urged outwardly from a fully retracted position essentially within the underreamer body to an opened position where a cam follower surface 55 engaged with a cam 49 are essentially parallel to the underreamer axis. The cam configuration provides only one opened position for the arm. Further outward movement of the arm is prevented by a stop plate 83 carried on the body which engages a stop projection 84 on the cutting arm. If it is desired to alter the expanded size of the Perkin underreamer, the stop plate 83 can be replaced by a stop plate having a different finger stop configuration so that the stop projections 84 will engage the radially inner faces of the stop plate fingers closer to or further from the underreamer axis. However, according to Perkin at col. 10, lines 19-27, it is also necessary to replace the entire cutter support arm by an arm having a different cam follower surface configuration.
It would be desirable to provide an underreamer where the amount of arm extension or its opening diameter can be adjusted without having to replace the entire arm or the configuration of the underreamer's arm actuating mechanism.